From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: passt.top; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: passt.top; dkim=pass (1024-bit key; unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256 header.s=mimecast20190719 header.b=artFWBBu; dkim-atps=neutral Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by passt.top (Postfix) with ESMTPS id 8F1115A0778 for ; Fri, 16 Jan 2026 04:25:19 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1768533918; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=vkpV5wm+rpcMcBIviFGL3O9cQmF5JgaWLLbbGqmcz4s=; b=artFWBBuD9rLrbBCaoWA6N9ArzUdTu0DjSA5KGVuMJQdr5fNLOJQEW9VZLTRFGXiEH/U0l dTVdpTvrabeodQZOKi8lUaHLDAWv4NoShsu0Rl2VO2IwSTws3aG1jF3DrzoyNYCNYi29cF t07sRlXieWeH+5AYn4fjhzgRZYZ61pU= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-613-ugp3lzceOH2p1Exbms9bzA-1; Thu, 15 Jan 2026 22:25:16 -0500 X-MC-Unique: ugp3lzceOH2p1Exbms9bzA-1 X-Mimecast-MFC-AGG-ID: ugp3lzceOH2p1Exbms9bzA_1768533915 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 95BC81800451; Fri, 16 Jan 2026 03:25:15 +0000 (UTC) Received: from fedora.nay.redhat.com (unknown [10.66.37.149]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id D46F21800285; Fri, 16 Jan 2026 03:25:12 +0000 (UTC) From: Yumei Huang To: passt-dev@passt.top, sbrivio@redhat.com Subject: [PATCH v2] conf, pasta: Add --splice-only option Date: Fri, 16 Jan 2026 11:25:09 +0800 Message-ID: <20260116032509.1480589-1-yuhuang@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: _TxUBMipK4ZSuwbaGloBJi48gJacVrIjFchdVvgmdUQ_1768533915 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: 8bit content-type: text/plain; charset="US-ASCII"; x-default=true Message-ID-Hash: RH6UPSFREWO3YZAN3TAFAAYQGP3GMB4U X-Message-ID-Hash: RH6UPSFREWO3YZAN3TAFAAYQGP3GMB4U X-MailFrom: yuhuang@redhat.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: david@gibson.dropbear.id.au, yuhuang@redhat.com X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: This patch introduces a mode where we only forward loopback connections and traffic between two namespaces (via the loopback interface, 'lo'), without a tap device. It might be used to fix up podman IPv4 / IPv6 loopback mapping when using rootlesskit for forwarding ports, or a way to implement isolated containers. In this mode, --host-lo-to-ns-lo and --no-icmp are automatically enabled. Option --no-splice is rejected. Link: https://bugs.passt.top/show_bug.cgi?id=149 Signed-off-by: Yumei Huang --- conf.c | 39 ++++++++++++++++++++++++++++----------- fwd.c | 3 +++ passt.1 | 5 +++++ passt.h | 2 ++ pasta.c | 3 +++ tap.c | 11 +++++++---- 6 files changed, 48 insertions(+), 15 deletions(-) diff --git a/conf.c b/conf.c index dbff87c..9d88ad7 100644 --- a/conf.c +++ b/conf.c @@ -1059,7 +1059,8 @@ pasta_opts: " --no-copy-addrs DEPRECATED:\n" " Don't copy all addresses to namespace\n" " --ns-mac-addr ADDR Set MAC address on tap interface\n" - " --no-splice Disable inbound socket splicing\n"); + " --no-splice Disable inbound socket splicing\n" + " --splice-only Only enable loopback forwarding\n"); passt_exit(status); } @@ -1142,7 +1143,7 @@ static void conf_print(const struct ctx *c) inet_ntop(AF_INET6, &c->ip6.addr_out, buf6, sizeof(buf6))); } - if (c->mode == MODE_PASTA) + if (c->mode == MODE_PASTA && !c->splice_only) info("Namespace interface: %s", c->pasta_ifn); info("MAC:"); @@ -1465,6 +1466,7 @@ void conf(struct ctx *c, int argc, char **argv) {"no-ndp", no_argument, &c->no_ndp, 1 }, {"no-ra", no_argument, &c->no_ra, 1 }, {"no-splice", no_argument, &c->no_splice, 1 }, + {"splice-only", no_argument, &c->splice_only, 1 }, {"freebind", no_argument, &c->freebind, 1 }, {"no-map-gw", no_argument, &no_map_gw, 1 }, {"ipv4-only", no_argument, NULL, '4' }, @@ -1961,8 +1963,11 @@ void conf(struct ctx *c, int argc, char **argv) } } while (name != -1); - if (c->mode != MODE_PASTA) + if (c->mode != MODE_PASTA) { c->no_splice = 1; + if (c->splice_only) + die("--splice-only is for pasta mode only"); + } if (c->mode == MODE_PASTA && !c->pasta_conf_ns) { if (copy_routes_opt) @@ -1971,6 +1976,16 @@ void conf(struct ctx *c, int argc, char **argv) die("--no-copy-addrs needs --config-net"); } + if (c->mode == MODE_PASTA && c->splice_only) { + if (c->no_splice) + die("--splice-only is incompatible with --no-splice"); + + c->host_lo_to_ns_lo = 1; + c->no_icmp = 1; + c->no_dns = 1; + c->no_dns_search = 1; + } + if (!ifi4 && *c->ip4.ifname_out) ifi4 = if_nametoindex(c->ip4.ifname_out); @@ -1994,9 +2009,9 @@ void conf(struct ctx *c, int argc, char **argv) log_conf_parsed = true; /* Stop printing everything */ nl_sock_init(c, false); - if (!v6_only) + if (!v6_only && !c->splice_only) c->ifi4 = conf_ip4(ifi4, &c->ip4); - if (!v4_only) + if (!v4_only && !c->splice_only) c->ifi6 = conf_ip6(ifi6, &c->ip6); if (c->ifi4 && c->mtu < IPV4_MIN_MTU) { @@ -2018,16 +2033,18 @@ void conf(struct ctx *c, int argc, char **argv) } if (!c->ifi4 && !v6_only) { - info("IPv4: no external interface as template, use local mode"); - - conf_ip4_local(&c->ip4); + if (!c->splice_only) { + info("IPv4: no external interface as template, use local mode"); + conf_ip4_local(&c->ip4); + } c->ifi4 = -1; } if (!c->ifi6 && !v4_only) { - info("IPv6: no external interface as template, use local mode"); - - conf_ip6_local(&c->ip6); + if (!c->splice_only) { + info("IPv6: no external interface as template, use local mode"); + conf_ip6_local(&c->ip6); + } c->ifi6 = -1; } diff --git a/fwd.c b/fwd.c index 961c533..6cdb197 100644 --- a/fwd.c +++ b/fwd.c @@ -771,6 +771,9 @@ uint8_t fwd_nat_from_host(const struct ctx *c, uint8_t proto, return PIF_SPLICE; } + if (c->splice_only) + return PIF_NONE; + if (!nat_inbound(c, &ini->eaddr, &tgt->oaddr)) { if (inany_v4(&ini->eaddr)) { if (IN4_IS_ADDR_UNSPECIFIED(&c->ip4.our_tap_addr)) diff --git a/passt.1 b/passt.1 index db0d662..8e8e179 100644 --- a/passt.1 +++ b/passt.1 @@ -755,6 +755,11 @@ Default is to let the tap driver build a pseudorandom hardware address. Disable the bypass path for inbound, local traffic. See the section \fBHandling of local traffic in pasta\fR in the \fBNOTES\fR for more details. +.TP +.BR \-\-splice-only +Do not create a tap device in the namespace. In this mode, \fIpasta\fR only +forwards loopback traffic between namespaces. + .SH EXAMPLES .SS \fBpasta diff --git a/passt.h b/passt.h index f3664ce..299185b 100644 --- a/passt.h +++ b/passt.h @@ -200,6 +200,7 @@ struct ip6_ctx { * @no_ndp: Disable NDP handler altogether * @no_ra: Disable router advertisements * @no_splice: Disable socket splicing for inbound traffic + * @splice_only: Only enable loopback forwarding * @host_lo_to_ns_lo: Map host loopback addresses to ns loopback addresses * @freebind: Allow binding of non-local addresses for forwarding * @low_wmem: Low probed net.core.wmem_max @@ -277,6 +278,7 @@ struct ctx { int no_ndp; int no_ra; int no_splice; + int splice_only; int host_lo_to_ns_lo; int freebind; diff --git a/pasta.c b/pasta.c index 0ddd6b0..bab945f 100644 --- a/pasta.c +++ b/pasta.c @@ -316,6 +316,9 @@ void pasta_ns_conf(struct ctx *c) die("Couldn't bring up loopback interface in namespace: %s", strerror_(-rc)); + if (c->splice_only) + return; + /* Get or set MAC in target namespace */ if (MAC_IS_ZERO(c->guest_mac)) nl_link_get_mac(nl_sock_ns, c->pasta_ifi, c->guest_mac); diff --git a/tap.c b/tap.c index 9d1344b..eaa6111 100644 --- a/tap.c +++ b/tap.c @@ -1491,13 +1491,16 @@ static int tap_ns_tun(void *arg) */ static void tap_sock_tun_init(struct ctx *c) { - NS_CALL(tap_ns_tun, c); - if (c->fd_tap == -1) - die("Failed to set up tap device in namespace"); + if (!c->splice_only) { + NS_CALL(tap_ns_tun, c); + if (c->fd_tap == -1) + die("Failed to set up tap device in namespace"); + } pasta_ns_conf(c); - tap_start_connection(c); + if (!c->splice_only) + tap_start_connection(c); } /** -- 2.49.0